The invention relates to the application of novel photopolymers based on specific urethane acrylates as writing monomers, which are suitable in the production of holographic media, in particular for the visual display of images, in printing processes.
Photopolymers are materials which can be exposed by means of the superposition of two coherent light sources, resulting in a three-dimensional structure in the photopolymers which can be described in general by a regional change of the refractive index in the material. Such structures are referred to as holograms. They may also be described as diffractive optical elements. The optical functions developed by such a hologram depends on the specific exposure to light.
For the use of photopolymers as supports of holograms for optical applications in the visible range, as a rule colourless or only very faintly coloured materials having a high diffraction effect are required after the exposure to light. Since the beginnings of holography, silver halide films, in particular those having high resolution, have been used for this purpose. Dichromate gelatine (DCG), dichromate salt-containing gelatine films or mixed forms of silver halide and DCG are also used. Both materials require a chemical aftertreatment for the formation of a hologram, which gives rise to additional costs for industrial processes and necessitates the handling of chemical developer solutions. Moreover, wet chemical methods result in swelling and subsequently shrinkage of the film, which may lead to colour shifts in the holograms, which is undesired.
U.S. Pat. No. 4,959,284 describes photopolymers which consist, inter alia, of a thermoplastic, such as polyvinyl acetate, cellulose acetobutyrate or polymethyl methacrylate-styrene copolymers, which are soluble in organic solvents, a photoinitiator and at least one vinyl cyclopropane derivative. EP352774A1 moreover describes monomers containing other vinyl groups constituted as N-vinylpyrrolidone, phenoxyethyl acrylate and acrylates of triols, such as trimethylolpropane (TMPTA) and ethoxylated trimethylolpropane (TMPEOTA), or other acrylates or acrylamides. It is known in the industry that such photopolymers show usable holograms only after relatively long thermal treatment. O'Neill et al. (Applied Optics, Vol. 41, No. 5, page 845 et seq., 2002), in their review article, discuss not only the abovementioned materials but also photopolymers which are obtainable from thermoplastics and acrylamide. In addition to the unfavourable toxicological profile of acrylamide, such products do not show bright holograms.
Holographically active materials which incorporate dyes which change their photosensitivity under the influence of light (Luo et al, Optics Express, Vol. 13, No. 8, 2005, page 3123) are also known. Similarly, Bieringer (Springer Series in Optical Sciences (2000), 76, pages 209-228) describes so-called photoaddressable polymers which are likewise polymer-bound dyes which can be isomerized under the influence of light. Holograms can be incorporated by exposure to light into both classes of substances, and these materials can be used for holographic data storage. However, these products are of course strongly coloured and therefore not suitable for the applications as described above.
Recently, photopolymers obtained not from thermoplastics but from crosslinked polymers were also described: US 020070077498 describes 2,4,6-tribromophenyl acrylate, which is dissolved in a polyurethane matrix. Likewise, U.S. Pat. No. 6,103,454 describes a polyurethane matrix with polymerizable components, such as 4-chlorophenyl acrylate, 4-bromostryrene and vinylnapthalene. These formulations were developed for holographic data storage, a holographic application in which many, but also very weak, holograms readable only by means of electronic detectors are written and read. For optical applications in the entire visible range, such formulations are not suitable.
Moreover, low-viscosity photopolymer formulations (DE102004030019, WO2005124456) which contain a difunctional acrylate with a mixture of natural unsaturated oils, a photoinitiator and an additive are described.
The non-prior-published PCT/EP2008/002464, EP 08017279.4, EP 08017277.8, EP 08017273.7, EP 08017275.2 disclose formulations of urethane acrylates as writing monomers in polyurethane matrices. Starting from these, it has now been found that printing of such photopolymer formulations is possible under certain circumstances.